A scroll compressor has been known as a refrigerant compressor used in a refrigeration cycle such as for refrigeration or air-conditioning, or a gas compressor that compresses air or other gases. Further, a compressor having a volume control mechanism has been known to achieve high efficiency over a wide load range. For example, one of volume control methods frequently used for controlling the volume of the compressor is such that a frequency of a motor current in the compressor is changed with an inverter to electrically control a rotational speed.
However, if the rotational speed is extremely reduced, a leakage of the refrigerant in a compression chamber may be increased and motor efficiency may be decreased, to substantially reduce compressor efficiency. Further, in an extremely low speed region of the rotational speed, an oil film at a bearing is not retained due to oil viscosity, to cause bearing elements to directly contact a shaft, leading to burning or the like. To prevent these problems, a lower limit value is set for a practical rotational speed and a lower limit is also set for a volume control range.
To extend the volume control range more than the lower limit of the rotational speed control, some control methods for controlling an amount of a refrigerant have been considered, in which compression is operated by mechanically bypassing a part of the refrigerant from the refrigeration cycle.
For example, Patent Document 1 discloses “a scroll compressor comprising a compression mechanism in a casing, in which at least one of a first scroll member and a second scroll member, each having on its end plate a spiral lap to engage with each other, eccentrically rotates and a compression chamber is defined by the engaged laps between the inner surfaces of the end plates; and an operation volume control mechanism having an opening that is formed in the end plate of the first scroll member and communicates with the compression chambers, and a piston that opens and closes the opening, wherein the piston is formed based on a dimension tolerance with which a top surface protrudes more than an inner surface of the end plate of the first scroll member in a state of closing the opening, and, at least a top portion of the piston, which protrudes more than the inner surface of the end plate of the first scroll member in the state of closing the opening, is formed of a material having an abrasion resistance lower than that of the laps” (see Claim 1).
In the structure above, the bypass valve (piston) is opened at the time of volume control, to communicate between the compression chambers and a suction side of the compressor via a bypass port of the opening. This bypasses the refrigerant without being compressed and delays a compression timing, to reduce a discharge flow amount of the compressed refrigerant for controlling the volume.